The present invention relates to loudspeakers, and more particularly to a loudspeaker designed to limit electro-acoustic transducer diaphragm excursion and to acoustically attenuate acoustic vibrations above a preselected frequency.
When a speaker is energized, its diaphragm reciprocates or vibrates at a frequency which varies with the signal input to the speaker. When an unmounted or unbaffled speaker is operated in a so-called "free air" mode, it exhibits large mechanical excursions as it approaches its resonant frequency. Significant acoustic distortion is often associated with this large mechanical excursion. This large mechanical motion continues to the resonant frequency and then falls off at higher frequencies. To control this motion and thereby reduce the distortion level of the speaker, it is customary to mount the speaker in some form of housing, so that the air in the housing will tend to control this motion.
In its simplest form this housing may be a closed box with the speaker mounted or suspended in an opening in one wall thereof. This construction causes the amplitude of an excursion to be lowered, and to occur at a different frequency, thus changing the resonant frequency of the speaker as compared to its "free air" mode of operation.
Another type of speaker housing is known as a bass reflex or ported enclosure. Typically this enclosure includes a hole or port in one of its walls, usually the wall or speaker panel upon which the speaker is mounted. The enclosure itself, as represented by the air therein, thus forms a resonator, and permits some of the air from within the enclosure to be driven or forced in and out of the port during vibration of the speaker diaphragm. Air can thus be considered to vibrate like a piston in the port, sometimes vibrating at the same frequency as the speaker diaphragm, and at times being out of phase with the diaphragm frequency. Ideally, however, the frequency of this air vibration is tuned to the resonant frequency of the speaker by proper sizing of the enclosure and the port. Loudspeakers of this bass reflex type are illustrated, for example, in U.S. Pat. Nos. 4,410,064 to Taddeo and 4,549,631 to Bose.
Bass reflex loudspeakers of the type disclosed in U.S. Pat. No. 4,549,631 to Bose, which utilize two subchambers having ports for directly acoustically coupling each of the respective subchambers with the exterior environment, tend to provide poor response for acoustic frequencies falling between the resonant frequencies of the two subchambers and their corresponding respective ports when the resonant frequencies of the two subchambers vary by more than a factor of 3 to 1. For instance, if the resonant frequency of the first subchamber and associated port is 50 Hz and the resonant frequency of the second subchamber and associated port is 250 Hz (a factor of 5 to 1), poor response is typically obtained for frequencies between these two frequencies, i.e. frequencies in the 100-200 Hz range.
Broadband loudspeaker systems often include separate loudspeakers for providing the low, midrange and high frequency components of the broadband acoustic signal. These separate loudspeakers are coupled together by a suitable crossover network for applying the appropriate frequency component of the electrical input drive signal to each of the loudspeakers. For maximum listening enjoyment, it is often desirable to limit the frequency passband of the acoustic output of each of the loudspeakers.
For instance, in broadband loudspeaker systems employing a subwoofer loudspeaker for generating the lowest frequency passband component of the broadband input signal, it has been accepted recently in loudspeaker design that localization can be inhibited, i.e. the placement of the subwoofer made unnoticeable, by restricting the subwoofer to operate up to a maximum frequency of about 150 Hz. Electrical filters have been used to restrict high frequency electrical drive signals from reaching the transducer of the subwoofer. Unfortunately, low frequency electrical drive signals, which are of course required to excite the transducer, can cause the transducer to generate higher frequency distortion products. Thus, electrical filtering of higher frequency electrical drive signals does not avoid the potential for localization.
Similarly, with separate loudspeakers for generating acoustic output signals corresponding to higher frequency bands of the electrical input signal, it is often desirable to limit the frequency of the acoustic output signals to a selected level. When such limitation is achieved by electrical filtering, distortion products can be generated in the same manner described above with respect to a subwoofer.